Schlenk flasks are round bottomed, while Schlenk tubes are
elongated. They may be purchased off-the-shelf from laboratory
suppliers, or made from round bottom
flasks or glass tubing by a skilled glassblower.

Contents

Evacuating a Schlenk
flask

Typically, before solvent or reagents are introduced into a
Schlenk flask, the flask is dried and the atmosphere of the flask
is exchanged with an inert gas. A common method of exchanging the
atmosphere of the flask is to flush the flask out with an inert
gas. The gas can be introduced through the side arm of the flask,
or via a wide bore needle (attached to a gas line). The contents of
the flask exit the flask through the neck portion of the flask. The
needle method has the advantage that the needle can be placed at
the bottom of the flask to better flush out the atmosphere of the
flask. Flushing a flask out with an inert gas can be inefficient
for large flasks and is impractical for complex apparatus. [1]

An alternative way to exchange the atmosphere of a Schlenk flask
is to use one or more "vac-refill" cycles, typically using a vacuum-gas manifold, also known as a Schlenk line. This
involves pumping the air out of the flask and replacing the
resulting vacuum with an inert gas. For example, evacuation of the
flask to 1 mm and then replenishing the atmosphere with 760 mm
inert gas leaves 0.13 % of the original atmosphere
(1/760 × 100%). Two such vac-refill cycles
leaves 0.000173% (i.e. 1/7602 ×
100%). Most Schlenk lines easily and quickly achieve a vacuum of 1
mm Hg.[2]

Varieties

When using Schlenk systems, including flasks, the use of grease
is often necessary at stop cock valves and ground glass joints to
provide a gas tight seal and prevent glass pieces from fusing. In
contrast, teflon plug
valves may have a trace of oil as a lubricant but generally no
grease. In the following text any "connection" is assumed to be
rendered mostly air free through a series of vac-refill cycles.

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Standard
Schlenk flask

A pear-shaped Schlenk flask. The flask's sidearm
contains a greased stopcock valve, and the flask is capped with a
Suba•Seal septum that has not
been turned down.

The standard Schlenk flask is a round bottom, pear-shaped, or
tubular flask with a ground glass joint and a side arm.
The side arm contains a valve, usually a greased stopcock, used to control the
flask's exposure to a manifold or the atmosphere. This allows a
material to be added to a flask through the ground glass joint,
which is then capped with a septum. This operation can, for example, be done
in a glove box. The flask can then be removed
from the glove box and taken to a Schlenk line. Once connected to
the Schlenk line, the inert gas and/or vacuum can be applied to the
flask as required. While the flask is connected to the line under a
positive pressure of inert gas, the septum can be replaced with
other apparatus, for example a reflux condenser. Once the
manipulations are complete, the contents can be vacuum dried and
placed under a static vacuum by closing the side arm valve. These
evacuated flasks can be taken back into a glove box for further
manipulation or storage of the flasks' contents.

Schlenk
bomb

A heavy walled, tube shaped, Schlenk bomb fitted
with a large bore plug
valve designed for high temperature closed system
reactions.

A "bomb" flask is subclass of Schlenk flask which includes all
flasks that have only one opening accessed by opening a teflon plug
valve. This design allows a Schlenk bomb to be sealed more
completely than a standard Schlenk flask even if its septum or
glass cap is wired on. Schlenk bombs include structurally sound
shapes such as round bottoms and heavy walled tubes. Schlenk bombs
are often used to conduct reactions at elevated pressures and
temperatures as a closed system. In addition, all Schlenk bombs are
designed to withstand the pressure differential created by the
ante-chamber when pumping solvents into a glove box.

In practice Schlenk bombs can perform many of the functions of a
standard Schlenk flask. Even when the opening is used to fit a bomb
to a manifold, the plug can still be removed to add or remove
material from the bomb. In some situations, however, Schlenk bombs
are less convenient than standard Schlenk flasks: they lack an
accessible ground glass joint to attach additional apparatus; the
opening provided by plug valves can be difficult to access with a
spatula, and it can be much
simpler to work with a septum designed to fit a ground glass joint
than with a teflon plug.

The name "bomb" is often applied to containers used under
pressure such as a bomb
calorimeter. While glass does not equal the pressure rating and
mechanical strength of most metal containers, it does have several
advantages. Glass allows visual inspection of a reaction in
progress, it is inert to a wide range of reaction conditions and
substrates, it is generally more compatible with common laboratory
glassware, and it is more easily cleaned and checked for
cleanliness.

Straus
flask

A Straus flask often called a solvent
bomb. "Solvent bomb" is a term that applies to any Schlenk
bomb dedicated to storing solvent. It is the construction of the
flask neck which makes a Straus flask unique.

A Straus flask (often misspelled "Strauss") is subclass of
"bomb" flask originally developed by Kontes Glass Company[3],
commonly used for storing dried and degassed solvents. Straus
flasks are sometimes referred to as solvent bombs — a name which
applies to any Schlenk bomb dedicated to storing solvent. Straus
flasks are mainly differentiated from other "bombs" by their neck
structure. Two necks emerge from a round bottom flask, one larger
than the other. The larger neck ends in a ground glass joint and is
permanently partitioned by blown glass from direct access to the
flask. The smaller neck includes the threading required for a
teflon plug to be screwed in perpendicular to the flask. The two
necks are joined through a glass tube. The ground glass joint can
be connected to a manifold directly or though an adapter and
hosing. Once connected, the plug valve can be partially opened to
allow the solvent in the Straus flask to be vacuum transferred to
other vessels. Or, once connected to the line, the neck can be
placed under a positive pressure of inert gas and the plug valve
can be fully removed. This allows direct access to the flask
through a narrow glass tube now protected by a curtain of inert
gas. The solvent can then be transferred through cannula to another
flask. In contrast, other bomb flask plugs are not necessarily
ideally situated to protect the atmosphere of the flask from the
external atmosphere.

Solvent
pot

A solvent pot ready to have its dried and degassed
contents vac transferred to another reaction vessel. This
pot contains dibutyl ether dried over sodium and benzophenone, which gives it its purple
color.

Straus flasks are distinct from "solvent pots", which are flasks
that contain a solvent as well as drying agents. Solvent pots are
not usually bombs, or even Schlenk flasks in the classic sense. The
most common configuration of a solvent pot is a simple round bottom
flask attached to a 180° adapter fitted with some form of valve.
The pot can be attached to a manifold and the contents distilled or
vacuum transferred to other flasks free of soluble drying agents,
water, oxygen or nitrogen. The term "solvent pot" can also refer to
the flask containing the drying agents in a classic solvent
still system. Due to fire risks, solvent stills have
largely been replaced by solvent columns in which degassed
solvent is forced through an insoluble drying agent before being
collected. Solvent is usually collected from solvent
columns through a needle connected to the column which pierces
the septum of a flask or through a ground glass joint connected to
the column, as in the case of a Straus flask.